On the Origin of the Non‐Arrhenius Na‐ion Conductivity in Na3OBr

Abstract The sodium‐rich antiperovskites (NaRAPs) with composition Na3OB (B=Br, Cl, I, BH4, etc.) are a family of materials that has recently attracted great interest for application as solid electrolytes in sodium metal batteries. Non‐Arrhenius ionic conductivities have been reported for these materials, the origin of which is poorly understood. In this work, we combined temperature‐resolved bulk and local characterisation methods to gain an insight into the origin of this unusual behaviour using Na3OBr as a model system. We first excluded crystallographic disorder on the anion sites as the cause of the change in activation energy; then identified the presence of a poorly crystalline impurities, not detectable by XRD, and elucidated their effect on ionic conductivity. These findings improve understanding of the processing‐structure‐properties relationships pertaining to NaRAPs and highlight the need to determine these relationships in other materials systems, which will accelerate the development of high‐performance solid electrolytes.


Electrochemical Impedance Spectroscopy (EIS)
The EIS characterisation was carried out in a custom-built PEEK solid-state cell using gold powder (Alfa Aesar, gold powder, spherical, APS 0.5 -0.8 µm) as the blocking electrodes. [1]Gold powder was added to one side of the cold-pressed pellet and then pressed at 370 MPa for 3 minutes before turning the pellet over to perform the same procedure on the opposite side.Using a screw on top of the cell, a uniaxial pressure of around 70 MPa was set by a torque wrench to ensure good contact.This pressure was calibrated using a load cell.The cell was then heated inside a muffle furnace in a glovebox for an hour at each data point before a measurement was performed using a two-probe configuration to a BioLogic MTZ35 frequency response analyser.Potentiostatic EIS (PEIS) measurements were conducted in the frequency range of 35 MHz to 0.1 Hz with a voltage amplitude of 10 mV.ZView software was used to simulate Nyquist plots based on the equivalent circuit depicted in Fig S1 .Ex-situ Scanning Electron Microscopy (SEM) SEM measurements were conducted using a Thermo-Fisher Helios G4-CXe Plasma FIB (PFIB) instrument with energy-dispersive X-ray spectroscopy (EDX) functionality (Oxford Instruments).Both cold-pressed (CP) and hotpressed (HP) pellets were mounted on a transportable SEM stage with carbon tape inside an Ar-filled glovebox before sealing in a Gatan iLoad sample transfer vessel.The stage was transferred from the vessel to the SEM chamber via a load-lock, avoiding exposure of the sample to air.

X-ray Diffraction (XRD)
Ex-situ X-ray powder diffraction measurements were performed on the as-synthesised Na 3 OBr powder using a Rigaku Smartlab diffractometer (Cu Kα) to assess its phase purity.Synchrotron XRD measurements were performed on the I11 beamline of the Diamond Light Source operating with an X-ray wavelength of 0.825 318(3) Å. Diffraction patterns were collected in capillary transmission geometry using the Mythen2 Position Sensitive Detector, two data collections of 5 seconds each were taken at angles 0.25 degrees apart, then summed to account for gaps in the detector coverage.Over the temperature range of 25 to 300 • C, patterns were collected at approximately 2.5 • C intervals with a continuous heating rate of 6 • Cmin −1 using an FMB Oxford cyberstar hot air blower.All Rietveld refinements were carried out using the TOPAS-Academic software. [2]In total, 20 parameters were refined: 11 polynomial function parameters to fit the background, the lattice parameter, b eq Na, b eq Br, b eq O, the scale parameter, and the 4 peak shape function parameters (pku, pkv, pkw, and pky).
Total scattering data were collected at I15-1 beamline at Diamond Light Source operating with an X-ray wavelength of 0.161 669 Å.Samples were put in 1mm borosilicate glass capillaries.Data for both background containers and samples were collected in a Q range from 0.3 to 25 A -1 and in a temperature range from 150 to 300 • C in 5 • C intervals.] Nuclear Magnetic Resonance (NMR) Spectroscopy NMR measurements were performed at room temperature and at variable temperatures.All the room temperature 23 Na static solid-state NMR measurements were completed at magnetic field strengths of 20 T (ν 0 23 Na = 264.6MHz) and 23.5 T (ν 0 23 Na = 224.9MHz) using Bruker Neo consoles and 3.2 mm probes.The spectra were referenced to NaCl at 7.2 ppm, [6] and were recorded using a Hahnecho (π/2τπτ ) sequence with 400 kHz WURST (wideband, uniform rate, smooth truncation) excitation and refocussing pulses for broadband excitation of the ≈ 220 kHz wide resonances (at 20 T). [7,8] The variable temperature NMR measurements were made at National Institute for Materials Science (NIMS) using an ECA-500 (JEOL, Japan) spectrometer and a homemade probe. [9]The sample was packed into a quartz NMR tube SP-405 (SHIGEMI, Japan) and sealed in an Ar-filled globe box.The resonance frequency of 23 Na was 132.32 MHz.The chemical shift was referenced to the 1.0 M NaCl solution at 0 ppm.The temperature was controlled by a nitrogen gas flow.

Figure S1 .
Figure S1.(a) Equivalent circuit used to simulate our experimental set-up.R 1 is the bulk resistance, CPE 1 is the bulk capacitance, R 2 is boundary resistance, and CPE 2 is boundary capacitance.(b) Nyquist plots of Na 3 OBr at multiple temperatures between 25 • C and 300 • C. Zoomed in plots from 230 • C to 300 • C can be found in the inset.(c) Arrhenius plot of boundary ionic conductivity of Na 3 OBr.(d) Arrhenius plot of bulk ionic conductivity of Na 3 OBr with a step increase observed at 250 • C. Details of the linear plots can be found in the insets.

Figure S2 .Figure
Figure S2.(a) Ex-situ X-ray diffractogram of Na 3 OBr.(b) Deviation of the lattice parameter of Na 3 OBr from a linear trendline as a function of temperature.
Figure S4.(a) Full-width half maximum and (b) integrated area under the impurities peak around 0 ppm as a function of temperature.

Figure
Figure S6.(a) Cross-sectional secondary electron image of the cold-pressed Na 3 OBr pellet before variable temperature EIS measurements.Corresponding EDX analysis of the different elements: (b) Na (purple), (c) O (red), and (d) Br (green).These images illustrate that Na, Br, and O are homogeneously distributed throughout the pellet.